Many products are sensitive to gases commonly found in the air, with oxygen tending to be the most problematic of these gases. For instance, many foods tend to be adversely affected by oxygen because they undergo chemical changes in the presence of oxygen that degrade their taste or color. One example of such a chemical change is the tendency of fats to react with oxygen and become rancid. Oxygen may also promote the growth of bacteria and the like which will cause food to spoil.
When commercially packaging oxygen-sensitive food products or the like for extended storage before sale or ultimate use by a consumer, care must be taken to minimize the product's contact with oxygen during storage. When canning food products, the cans are hermetically sealed to keep air from coming into contact with the food. In commercial canning processes, the containers tend to be formed entirely out of metal, with seams being welded or mechanically crimped closed. In home canning processes and in some other commercial processes, glass containers having metal lids are used and the lids commonly include a compressible sealing strip of a rubber-like material to form an air-tight seal between the glass jar and the lid. In some home canning processes, an additional wax barrier is placed between the product and the food to limit contact with any oxygen which may leak through the seal between the jar and the lid.
Both of these types of containers tend to be quite effective in keeping oxygen out of the interior, and hence out of contact with the food stored inside, because metal and glass are essentially absolutely oxygen-impermeable. So long as the seal between the component parts of the containers remains intact, there is little chance that oxygen will enter the container and affect the food.
In modern packaging, plastic materials have in many instances entirely replaced metal or glass as the primary component of the package due to the lower cost of plastics. For instance, frozen pizzas and high-fat products such as potato chips and the like are commonly sold in an entirely plastic container, perhaps using a label formed of paper or some other readily printed material. Unlike metal or glass, though, virtually all polymeric materials used in packaging food are at least slightly permeable to oxygen, with the permeability varying from one plastic material to another. Although plastics tend to be significantly cheaper than metal or glass in most food packaging applications, the oxygen permeability of plastic films can also reduce the effective shelf life of the product contained in the package.
Many attempts have been made to develop materials for use in the packaging industry that minimize oxygen transmission; these attempts have encompassed both development of homogenous polymeric films with new plastics and composite films that may include layers of different plastics. Among the polymeric films most commonly used in the packaging industry are polyvinylidene chloride (PVDC, sold under the trade name "Saran"), which has a relatively low gas permeability or transmittance; biaxially oriented nylon, which exhibits moderate oxygen transmittance; and polyethylene, which transmits oxygen more freely. For instance, a 13 micron film of a PVDC will transmit about 4.0 cm.sup.3 of oxygen/m.sup.2 of surface area/atmosphere/day, while a 1 mil (25 micron) film of nylon 6 will transmit oxygen at about ten times that rate (about 40 cm.sup.3 of oxygen/m.sup.2 of surface area/atmosphere/day).
Current composite materials may include a layer of a metal foil, e.g. a 25 .mu.m aluminum foil, disposed between a pair of plastic films, which may be formed of different polymers if so desired. One of the advantages of using a plastic/metal composite material is that the metal layer can, if thick enough, make the composite material substantially totally oxygen impermeable.
Unfortunately, though, materials which provide better resistance to oxygen transmission also tend to be more expensive. Plastic/metal composite films are generally much more expensive than a film formed solely of the plastic material and are also opaque, preventing a consumer from seeing the contents of the package at the point of sale. There are also significant cost differences between different polymeric film materials. As a general rule of thumb, polymeric films which have lower rates of oxygen transmittance tend to be more expensive than films with higher oxygen transmission rates.
The oxygen transmittance of a polymeric film of a given composition is generally inversely proportional to its thickness--a film which is twice as thick will transmit about half as much oxygen. Polymeric films used as walls of containers also have to meet certain other physical requirements, such as minimum tensile strength, to provide a suitable commercial container. Accordingly, it is frequently more cost-effective to use a thicker film of a cheaper plastic material than a thinner film of a more expensive plastic to achieve the same net oxygen transmittance.
Numerous attempts have been made to provide a more cost effective oxygen barrier. For instance, U.S. Pat. No. 4,105,818, issued to Scholle, sets forth an alleged improvement in packages using polymeric films. According to Scholle's teachings, one can improve the barrier properties of a plastic film by splitting a single thicker ply of plastic into a pair of thinner films, with each of the thinner films having a thickness about half that of the thicker film. Scholle claims that, at steady-state conditions, a single ply of 0.5 mil PVDC transmits about twice as much oxygen as a single ply of 1.0 mil PVDC, (as one would expect), yet a composite film consisting of two plies of 0.5 mil PVDC transmits less than half the oxygen transmitted by the 1.0 mil film.
This is counterintuitive in that one would expect such a film to transmit at about the same rate as the 1.0 mil film since its total thickness is the same. As explained more fully below, though, this simply is not believed to be the case.
At steady-state, the two-ply film will indeed transmit oxygen at substantially the same rate as a single-ply film having the same total thickness. Furthermore, if air is permitted to remain between the two plies of the composite film it has been found that the transmission rate of such a composite film is actually substantially greater than that of the single-ply film of the same thickness, at least initially. Only after some time has passed will such a composite film approach the transmission rate of the single-ply film of the same thickness; such a film cannot reduce the transmission rate below that of a single-ply of the same thickness.
Other attempts have been made to extend the shelf life of oxygen-sensitive films by providing "oxygen absorbing" materials in the container with the product. Such oxygen absorbing materials operate on the principle that they are more reactive with oxygen than the product and therefore will consume oxygen entering the package before it can react with the food product. For instance, an oxygen absorbing product sold by Mitsubishi Gas Chemical Company under the trademark AGELESS utilizes finely divided iron powders to scavenge oxygen from the atmosphere.
However, there are concerns with placing such a material in direct physical contact with food products. In addition to obvious risks of degradation of taste and color of the foods, there are concerns regarding the possible reactions of these powders with the food products themselves. Accordingly, if such oxygen absorbing products are used in packaging foods, they must generally be physically isolated from the food. This adds further complexity, and hence cost, to these packages.
It would be advantageous to provide a cost-effective polymeric film material for packaging oxygen-sensitive products which does not suffer from the problems associated with prior art materials. In particular, such a film is desirably translucent or substantially transparent to permit consumers to see the food products at the point of sale. It should not introduce potential contaminants into the inner cavity of the container wherein the product is stored. And, perhaps most importantly, it should provide an effective barrier to oxygen transmission to enhance the shelf life of oxygen-sensitive products without unduly increasing the expense of the package.